Dear Sirs,
please advise for the follwing design condition:

Code : Div 1
Design Temp = - 50 Deg C
Operating Temp = -45 Deg C
MDMT = -50 Deg C

But, as per UG-20, Design temp should be greater than operating temp.
Whether this clause is applicable for Lower temp. or not?

As per this clause, for the above condition my design temp. should be morethan -45 Deg C. (i.e. -40 or -35 or -30 or ......................)

Can we apply UG 20 clause for the negative tempatarutes or not? if so,

Please advise when a vessel designed at -10 Deg C can be operated at -45 Deg C or not? And logically howfar it is correct?

Does the code refers this any where?

Please advise

Regards

Srinivas

Hello Srinivas,

Indeed, according to UG-20(a) the "Design Temperature" shall not be less than the "mean metal temperature expected under operation conditions".

Regardless of that you have to check the MDMT at MAWP, independent from the "normal design".

Besides, why do you perform a design calculation for -50°C? I don´t expect a reduced allowable stress value at 10°C . Moreover, in the first case you are not allowed to set your vessel under pressure before cooling down to -50°C, even during the hydro if you haven´t checked this, does this makes sense .

As mentioned above you can operate your vessel when you have checked the MDMT (here -45°C) at MAWP. The stress calculation and the MDMT examination are two different procedures.

Michael

Srinivas,

UG 20 (a) "Design temperature shall not be less that the mean metal temperature" to be taken in its real essence and not the literal meaning. Clasue is written for the worst design condition.

For eg, if your operating temperature range is -45 to +45 deg C, then for positive temperature you have to add some margin on it (say 10 deg C for design) then it becomes 55 deg C (45 +10) similarly for negative temperature, you have to consider some margin (say -5 deg C) then minimum design temperature becomes -50 deg c (-45 - 5). Do not take the term "HIGHER" in its literal meaning. -50 is higher/ worst than -45 when you refer to negative temperature.

MDMT shall affect material selection and the impact testing/ PWHT requirement however for other strength calculations it will not have any impact. MDMT to be established in the worst case of operating case, not necessarily at MAWP (for high pressure gas service equipment, blow down may be a worst case to establish MDMT).

Sanjeev